Field of the Invention
The present invention relates to an image forming apparatus for forming images on sheets.
Description of the Related Art
Hitherto, an image forming apparatuses such as copying machines, facsimiles and laser printers capable of forming images on a first side (front surface) and a second side (rear surface) of a sheet by using electro-photography system have been provided. When forming images on both sides of a sheet in such image forming apparatus, a toner image is formed on a photosensitive drum, then the toner image is transferred at a transfer portion to a first side of the sheet supplied from a sheet feed portion, and then the toner image is fixed to the sheet at a fixing portion. Thereafter, the sheet having the image fixed to the first side is reversed and conveyed again to the image forming portion via a re-conveyance path, to have an image formed on the second side of the sheet.
Generally, in the conventional image forming apparatus, the time required from starting an image forming operation such as by pressing a copying button to discharging the sheet on which images have been formed on a discharged sheet tray (hereinafter referred to as FCOT) should preferably be as short as possible from the viewpoint of the user. The same applies to the case where duplex image forming is performed.
However, in an apparatus designed so that the re-conveyance path allows images to be formed on both sides of a sheet having a large size, such as A3 size, the re-conveyance path is set long to correspond to the large-sized sheet. When performing duplex image forming on small-sized sheets such as A4-sized sheets using the re-conveyance path, the FCOT during duplex printing becomes undesirably long.
For example, Japanese Patent Application Laid-Open Publication No. 2006-298605 discloses an apparatus having a reverse conveyance path for small-sized sheets for reversing small-sized sheets and a re-conveyance path for re-conveying the small-sized sheets being reversed by the reverse conveyance path toward the image forming portion within the apparatus, with the aim to shorten the FCOT. Further, when re-conveying a large-sized sheet again to the image forming portion on the image forming apparatus, the sheet is conveyed so that a part of the sheet is protruded to the exterior of the apparatus, and then reversed.
Further, Japanese Patent Application Laid-Open Publication No. 02-008167 discloses an image forming apparatus designed to stack reversed sheets on a re-feeding tray. In the disclosed image forming apparatus, a reverse path for small-sized sheets for stacking the reversed small-sized sheets to a re-feeding tray is disposed downstream of a fixing device, and a reverse path for large-sized sheets is disposed downstream of the reverse path for small-sized sheets, with the aim to shorten the FCOT.
However, according to the image forming apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2006-298605, a reverse conveyance path only for reversing the small-sized sheets is disposed within the apparatus, and as a result, the size of the main body of the apparatus is increased.
On the other hand, regarding the image forming apparatus taught in Japanese Patent Application Laid-Open Publication No. 02-008167, separate reverse conveyance paths are disposed for reversing small-sized sheets and large-sized sheets respectively, so that the size of the main body of the apparatus is increased as a result. Even further, since the sheet having an image formed on the first side is temporarily stacked on the re-feeding tray before being fed to the image forming portion again, the FCOT during duplex printing is elongated and the productivity is deteriorated.